I've played both, but when I played WS it was in 2004 before the PGA and the maximum fairway width was 22 yards wide. Whistling was letting the grass grow in for the PGA to shape it as they pleased. Also, it was soaking wet with no roll and I played the back tees.
Factoring all this, from my experience, I think that Kiawah is harder on a normal day from the 6800 tees. Kiawah had much wider fairways than what I saw at WS, but the wind and short game challenge at Kiawah are much more to contend with.
Mike, you're coming from Kiawah....not much is going to be tougher than that!!